ance both directly and indirectly. Direct effects include the change of albedo and emissivity resulting from the different types of land covers that modify the amount of shortwave radiation absorbed at the surface and of longwave radiation absorbed and emitted at the surface. For example, the development of agriculture in tropical regions typically results in an increase of albedo from a low value of forest canopies (0.05-0.15) to a higher value of agricultural fields, such as pasture (0.15-0.20). In contrast, irrigated fields in arid areas tend to have a lower albedo than the bare, typically bright soils they cover. The seasonal variation of albedo as a result of land-cover change can also have pronounced effects on the net radiation at the Earth’s surface.
As shown in Figure 2-1, the IPCC (2001) reports the global-averaged forcing due to albedo change alone as −0.25 ± 0.25 W m−2. The level of scientific understanding is listed as “very low.” The uncertainties in the albedo change reflect the complexity of the land surface (e.g., type of vegetation, phenology, density of coverage, soil color). When aggregating regional information about land surface up to the global scale, large global average uncertainty ranges result. A recent assessment of the albedo change estimates a range of −0.6 to 0.5 W m−2, with the negative values being more likely (Myhre and Myhre, 2003).
Indirect effects of land-cover change on the net radiation include a variety of processes related to (1) the ability of the land cover to use the radiation absorbed at the ground surface for evaporation, transpiration, and sensible heat fluxes (the impact on these heat fluxes caused by changes in land cover is sometimes referred to as thermodynamic forcing); (2) the exchange of greenhouse and other trace gases between the surface and the atmosphere; (3) the emission of aerosols (e.g., from dust); and (4) the distribution and melting of snow and ice. These effects are discussed below.
Changes in soil wetness can significantly modify the energy balance of continental surfaces. When soil moisture is high, most of the radiative energy absorbed at the ground surface is used for physical evaporation and transpiration of water. The latent heat flux is large, the sensible heat flux is small (in arid areas, it can even be negative, a process known as the “oasis effect”), the land-surface temperature is relatively low (compared to conditions with more sensible heat flux with the same net radiation), and as a result, the longwave radiation emitted by the land surface is relatively low. As a result, the atmospheric boundary layer that develops above such land is typically thin and moist. In contrast, when the soil is dry, there is no latent heat flux, the sensible heat flux is large, the land surface temperature is higher, and as a result, the longwave radiation emitted by the land surface is relatively high. The planetary boundary layer developing above such land